1. Field of the Invention
This invention relates generally to testing a conductor path for continuity and, more particularly, to apparatus and associated methodology for detecting an open in a multipair cable conductor.
2. Description of the Prior Art
In the subscriber loop portion of telecommunications systems, shielded multipair cable is presently the predominant medium utilized for signal transmission. The cable generally comprises many individually insulated conductors twisted together into pairs; the pairs are contained within a single protective sheath which includes a continuous metallic layer. Typically, each twisted pair connects a customer premises to a switching point, usually a central office, or numerous pairs connect switching points.
In providing for new transmission facilities, or in rearranging old facilities, it is oftentimes necessary to join or splice together a number of cable segments to establish an overall cable route. In these situations, testing for defective splices is effectuated after the splicing activity to insure reliable completion of the splicing operation.
The splicing activity involves the step of electrically interconnecting predetermined wires from one cable segment with companion wires from an adjacent cable segment so as to form a DC short at the junctures of the wire segments. During this splicing step, wires may be rendered defective in a number of ways. For instance, a wire may be inadvertently broken and the break may not be visually perceptible due to the insulating coating on the wire. If a crimp-type connector is utilized to join the wires, a faulty connector may not effect a short between wires or a malfunctioning crimping tool may not apply sufficient pressure to form a solid short. These types of faults preclude using the wire because of the service-affecting nature of the fault.
To detect an open or faulty splice, various techniques and equipment have been developed. However, each method has at least one major shortcoming.
The most typical prior art fault-detecting technique is that of measuring the DC resistance of the spliced wires on an end-to-end basis. Since the far end points of the wire segments are generally not available at one geographical location, the operation involves two-ended testing. This procedure can be expensive and time-consuming because the two-ended nature of the technique generally requires personnel at both ends to manipulate apparatus. Moreover, other faults in the cable segments may interfere with the end-to-end testing and yield meaningless test results.
Other prior art techniques for testing an electrical path for conductivity whenever the other ends of the path are spaced apart have focused on monitoring the local path comprising the splice itself and the portion of the wire segments proximate the splice.
Representative of these localized testing techniques is the subject matter of U.S. Pat. No. 3,344,348, issued to I. M. McNair, Jr. et al on Sept. 26, 1967. The McNair et al invention discloses apparatus to monitor splice resistances during a so-called cut-closed cable operation, that is, the operation of transferring a pair from a portion of an old cable to a new cable portion that is remotely bridged to the old cable. One step in the operation calls for insuring that the final splice joining each conductor from the new pair to each conductor from the remaining portion of the old pair is electrically a short. Two test set leads, both directly connected to the same conductor and proximate each other, straddle the point of cutting of the old wire and the eventual point of splicing together the old and new conductors. A cut causes a large impedance in the local series path comprising the two test set leads and the short piece of conductor between the leads, whereas a good splice effectively shorts the leads. Circuitry coupled to the leads monitors for the alternating open-short conditions and signals accordingly.
A major shortcoming with these techniques is the necessity of making direct metallic contact with the individual wires or conductors. This contact is typically effected with an insulation-piercing tool. Since many splice points are not moisture-proofed or hermetically sealed after the splicing operation, preservation of insulation integrity is critical to avoid corrosion, leakage and other failures that cause service-affecting or service-degrading transmission.
Another limitation of directly connecting test apparatus to the metallic conductor occurs in applications involving splicing operations on working or in-use cable pairs. A direct connection to the DC path generally causes a "hit," that is, a transient disturbance caused by modifying the impedance of the propagation medium. A hit is particularly troublesome for high-speed transmission due to the susceptibility of the signal to high-frequency interference.